The object of the present invention is a mortar composition, which can be cured after thermal initiation by frontal polymerization, as well as a method for fastening tie bars, reinforcing steel or the like in solid substrates using this mortar composition.
In order to fasten tie bars, reinforcing iron or similar elements in a borehole in mineral substrates, such as concrete or masonry, two-component mortar compositions based on methacrylate or epoxide resins are normally used. After the mutually reacting components are mixed, these mortar compositions have a certain pot life, during which the element, which is to be fastened, can be set, and reach their final strength at the expiration of a further time period. Under usual conditions, said pot life lies within the range of a few minutes. Normally, curing takes place within a few minutes to hours. In every case, both effects are interlinked, that is, a longer pot life leads to a longer curing time, and these times can vary depending on the surrounding conditions, particularly the temperature.
On building sites, where such fastening elements usually are set, it is hardly possible to work under optimum conditions. Therefore, when a plurality of boreholes, for example, initially are filled with a mortar composition and the fastening elements are then introduced one after the other, different time periods elapse between the mixing of the curing mortar mixture and the introduction of the fastening elements. This can lead to premature curing of the mortar composition, so that the borehole no longer can be used. This is critical, above all, at high temperatures (in the summer).
There is therefore a need for a mortar composition, which has a very long pot life and the curing of which can be initiated selectively at a desired time. By these means, it became possible initially to provide a large number of boreholes with the mortar composition, subsequently to introduce and adjust the fastening elements and then to initiate the curing, as a result of which it becomes possible to attain an optimum and largely identical curing and, with that, largely identical pull-out strengths of the fastening elements that have been mounted.
The free radical frontal polymerization has already been described by J. A. Pojman et al. (J. Chem. Soc. Faraday Trans. (1996), 2825-2837). It is stated that, under certain conditions, the heat, released by a polymerization reaction, can be used to stimulate the polymerization of adjacent monomer regions. It is, however, stipulated that the frontal polymerization takes place with good thermal insulation and that the front, in order to avoid interfering convection effects, spreads out downward because of gravity. The synthesis of materials as well as the curing of large volumes of composite materials is described as the purpose for which frontal polymerization is used.
The object of the present invention is to provide a mortar composition, with which it is possible to separate the pot life from the curing time of the mortar composition and, with that, to place the fastening elements into the boreholes optionally even hours after the mortar composition is introduced, to align them and then by thermal initiation to bring about the curing within seconds up to a few minutes, so that it is readily possible, even under the heat-conducting conditions of the chemical anchoring of primarily metallic fastening element in mineral substrates, such as concrete, masonry and the like, to bring about a selective curing of the mortar composition in the dowel holes of fastening elements. It is a further object of the present invention to provide a method for using such a mortar composition to fasten fastening elements, especially tie bars, reinforcing steel or the like, in solid substrates.
These objectives are accomplished by the mortar composition of claims 1 to 17 and the method for fastening tie bars, reinforcing steel or the like in solid substrates using this mortar composition is accomplished in accordance with claims 18 to 21.
The object of the invention therefore is, in particular, a mortar composition, which can be cured after thermal initiation by frontal polymerization and is characterized by containing
a) at least one polymerizable monomer and/or at least one curable resin,
b) at least one polymerization initiator, which can be activated and/or released thermally at a temperature above 30xc2x0 C., for the polymerizable monomer and/or curing accelerator for the curable resin and
c) optionally at least one filler, the nature and amount of polymerizable monomer or curable resin and polymerization initiator or curing accelerator being selected so that a polymerization front velocity of at least 10 cm/min results after the polymerization is initiated.
The inventive mortar composition cures only when, due to thermal initiation, that is, due to pointwise or two-dimensional heating of the surface layer or by heating the interior of the mortar composition to a temperature above 30xc2x0 C., the polymerization initiator or curing accelerator, which can be activated and/or released thermally, is activated and the polymerization of the monomer and/or of the curable resin is initiated. By these means, it becomes possible to attain a mortar pot life of practically any length and to separate this pot life completely from the curing time, since the curing sets in only as the result of the thermal initiation. In this way, it becomes possible to set and adjust the element, which is to be fastened, even hours after the mortar composition is introduced into the borehole and to bring about the curing of the mortar composition within seconds up to a few minutes by briefly heating the surface of the mortar.
Moreover, the inventive mortar composition enables fillers, which are required in order to attain high load values, to be incorporated. At the same time, the surprisingly high frontal velocity of the inventive mortar composition of at least 10 cm/min is not affected by the use of fillers.
Finally, with the inventive mortar composition, it is possible, not only to bring about the frontal polymerization downwards in the direction of gravity, but also in a horizontal or vertically upward direction. In this way, by appropriately adjusting the viscosity of the mortar composition, it is possible to fill boreholes, which are present in ceilings and open up downward, with the mortar composition, to introduce the fastening elements and to trigger the curing by thermal initiation.
Pursuant to the invention, the initiation of the polymerization of the mortar composition takes place here preferably by pointwise or two-dimensional heating of the surface layer with the help of a flame, a soldering iron tip, a hot air blower, a flash/laser beam, an induction furnace or by electrical resistance heating, for example, with the help of a hot wire of a resistance heater or the like, or in situ by a chemical reaction or by introducing heat into the interior of the mortar composition over a heat-conducting fastening element.
The frontal velocity, that is, the velocity of the polymerization front moving through the mortar composition, is determined by measuring the course of the temperature in the mortar composition in a defined annular gap as a function of time. The exact determination of the frontal velocity will still be explained in greater detail below.